Seismograph gain control system



Jan. 24, 1950 E. J. sHlMEK 2,495,390

SEISMOGRAPH GAIN CONTROL SYSTEM Filed March 2o, A194e v l 2 sheets-sheet 1 1 vvvvvv BYM Jan. 24, 1950 E, J, $|||MEK 2,495,390

'ZSEIsMosRAm GAIN CONTROL 'SYSTEM Filed March 2o', 1194s' '.2 sheets-sheet 2 Vicki INVENToR. b/z//An J JH/Mf/r BY gw Patented Jan. 24.' 1950 J 'f SEISMOGRAPVH GAIN CONTROL SYSTEM Edwin J. Shimek, Dallas, Tex., assignor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application March 20, 1946, Serial No. 655,659

, 2 Claims.

The present invention relates to a control circuit for amplieraand more particularly to a signal control circuit for use with electric seismographs.

In seismic prospecting systems, it is customary to detonate a charge of explosive in a shot hole to produce artificial seismic waves which are reflected from sub-surface horizons or interfaces. As these waves travel through the underground strata, they are attenuated so that the waves reflected from the deeper interfaces are of lesser amplitude. The attenuation of the waves depends upon the distance traveled and also upon the different densities in the sub-surface strata.

The reflected seismic waves are picked up at a plurality of'spaced points remote from the shot hole by detectors or geophones arranged in a pattern generally referred to as a spread The reflected Waves reach the different geophones at different times, and different interfaces cause a different series of reflections to reach the geophones so that a recording of the reception of such waves by a seismic oscillograph will provide data from which computations may be made to plot the sub-surface strata and contour of the interfaces.

The time of arrival of the rst wave is ascertained by the beginning of the movement of the oscillograph element from its neutral position. In order to facilitate determination of the initial movement of the oscillographic element with reasonable accuracy, it has been customary to have theamplifler set for maximum gain. With the amplifier set for maximum gain, the initial movement of the galvanometer or oscillographicA element produces a sharp break in the seismogram, thereby minimizing the necessity to estimate the precise instant at which the oscillographic element first began to move.

Subsequent to the initial direct wave, a series of reflected waves are received which over a period of time are progressively attenuated. The rst reflected waves are of higher amplitude than succeeding reflected waves, so that it has been customary to reduce the gain of the amplifier for the rst reflected Waves and subsequently to increase the gain ofthe amplifier as a function of time. It has been found, however, that increasing the zgain of the amplifier as a function f time by a modified gain control circuit operating in a manner analogous to theautomatic volume control of radio receivers introduces a modulation or distortion of the received waves.

In radio receiver, the audio or radio frequencies are at a frequency which is relatively high compared to the low frequency of operation ofthe automatic gain control voltage. In seismographic recording, however, the seismic signals are quite low in frequency and are comparable to the undesired signal inherently resulting from variation of the gain control voltage. In radio receivers, a simple filter arrangement will substantially eliminate the unwanted signal from the output, but a simple filter circuit will not eliminate the undesired signal from the amplifier output in a seismographic apparatus. It, therefore, would be desirable to provide a signal control for the amplifier which would provide a wide variation of the control of the amplitude of the signal to be recorded with minimum modulation of the signal control voltage.

In accordance with the present invention, an electronic attenuation or signal control is obtained for operation in connection with a seismographic amplifier by means of a circuit which Varies the magnitude of'signals transmitted thereby Without greatly affecting the potentials applied to the amplier circuit. This is accomplished by the use of a symmetrical circuit which may be unbalanced by a small control voltage so as to control the signal amplitude from a limiting value of zero to the maximum.

It is, therefore, an object of the present invention to provide an improved control circuit for controlling the amplitude of transmitted signals.

It is a further object of the present invention to provide an improved signal transmission circuit for controlling the amplitude of signals in a seismographic recording system. y

Other and further objects of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which:

Fig. l is a circuit diagram of a signal control circuit constructed in accordance with the present invention;

Fig. 2 is a graph explanatory of the mode of operation; and

Fig. 3 is a circuit diagram of a modiiied form of the invention.

Referring to Fig. 1 of the drawing, there is shown a circuit having an input transformer lil, the primary winding ll of which is connected to a source of seismic signals such as those picked up by a geophone l2. The secondary winding I3 of the transformer lll has its terminals capacitively connected by capacitors I4 and l5 to the control grids of a pair of vacuum tubes I6 and Il. The secondary Winding of the transformer l0 may be bridged by afresistor I8 vso as to complete a circuit for the currents of the secondary winding i3 and properly to match the transformer primary to the impedance of the geophone I2, The voltage developed across the winding I3 is applied to a resistor IQ connected between the control grids or the vacuum tubes I5 and Il'.

The pair of vacuum tubes It and Il may be any suitable vacuinn tubes although it has been found preferable for the present purposes to utilize pentodes having their suppressor grids connected to the cathodes. The cathodes are self-biased by resistors 2i and Z2 connected to ground and by-passed respectively by capacitors 23 and 24. An adjustable contact on the resistor I9 is yconnected to the common juncture 'of ythe cathode bias resistors 2l and 22. rlhe adjustable contact on the resistor I9 is provided for accurate balancing of the circuit and to provide the effect of a midpoint for the secondary winding I3 of the transformer It. A source of potential 25 has its negative terminal connected to ground-and tothe cathodes of the vacuum tubes. An intermediate point on this source of potential 25 is connected to the screen grids of each of the vacuum. tubes I6 and .I "I. The source of potential 25 isconnected to anode resistors 2B and l2l which complete the anode circuits for the vacuum tubes It and Il. respectively. IThose skilled in the vart will recognize from the circuit thus far described a certain similarity of push-pull amplifiers of the resistn ance-capacitance coupled type.

The anodes of the vacuum tubes I6 and il are coupled .by capacitors 28 and 29 .to one diagonal of a bridge network formed of suitable impedances such as resistors 3l, 32, 33 and 34. The resistors 33 and 34 may be combined in a single resistor having an adjustable contact 35 which is connected to ground. The common juncture between the resistors 3I and.32 is connected to the control grid of a vacuum tube 3B. The cathode of .the vacuum tube 36 may be .selfbiased by a resistor .31 connected to ground and by-passed `by a capacitor 38. The vanode of :the vacuum tube 36 is connected through an .anode resistor 39 to the source ofpotentialz.

The output of the vacuum ,tube v35 is capacitively coupled by a capacitor 4l to an input .resistor 42 which may be connected to succeeding stages of ampliiication indicated at AM and which lead to the yrecordingiseismograph R. The upper terminal of the resistor 42 .is `connected to one terminafl oi the primary winding 43 of .a transformer e4. The other vterminal .of the primary winding of the transformer -44 is connected to ground. The transformer -44 lis provided with a lmidtap secondary winding I5 having its outer extremities connected to the anodes of a pair of rectiers 4E and 41. Thecathodes of the rectifers are connected together Yand to one kterminal of a resistor 48, the other terminal of which is [connected to the midpoint of the secondary ywinding 45.

The output of the rectiers 46 and 4'! is connected to a resistance-capacitance lter circuit comprising the resistor 49 having at each terminal oy-pass filter capacitors 5I and `52. The cathodes 'of the vacuum tube rectiers 46 and 41 are connected through the .resistor 49 to one terminal of the resistor I9 of Ythe input circuit of the pair of vacuum tubes It and I1. Theother terminal of the resistor I9 is connected through a source of potential V53 to the midpoint of the transformer secondary winding 45. The source of potential 53 is applied to the grid circuits of the vacuum tubes 16 and I'I in vseries with resistor IS so as to unbe-lance these vacuum tubes in opposite senses.

A portion of the output of the amplier tube 35 is supplied to the rectiers 45 and 4'! to produce rectified energy proportional to the signal output. This rected energy iows through the resistor `49 developing Va potential `thereacross which is in opposition -to the Vpotential provided by the source of potential of battery 53. This therefore provides an operation which tends to reduce the amount of unbalance produced by the battery 53 in the-operation of the vacuum tubes -IE and Il'.

If vno bias potential were supplied from the `rectiiier-iilter network to the grid circuits of the vacuum tubes i5 and I'I, it will be seen that the vacuum tubes would operate in a balanced manner corresponding to the balanced operation of Class `A .push-pull amplifiers. Under such an assumed condition of operation, if the current through one of the resistors ysuch as -25 should increase, `the current through the other resistor 2?! would decrease. The increase and decrease would be at comparable rates so that the net change in current flow through the conductorfto the battery 25 would `be zero. Substantially iniinite attenuation is thereby provided and it vfollows that any unbalance, no matter how small, will produce an output signal.

More particularly, if the operating conditions o the vacuum tubes -Iii kand IFI are unbalanced, the flow -of currents ythrough `the resistors 26 and 2l will not be increasing and decreasing at comparable rates so that an alternating-current component will flow through the conductor extending between the common juncture of the resistors and uthe battery 2,5. Thus, the signal or alternating-current lvoltages vdeveloped across resistors 2,6 and 21 will be dissimilar and the voltage applied. to the points A and B by ythe capacitors `28 and 2S relative to ground will be of diierent magnitudes, with resulting current flow through the arms of .the bridge network. For example, .if the alternating-.current :potential developed across the .resistor 2 6 relative to ground is greater than the opposite potential developed across the resistor 2l, a greater .amount of currentfwill flow Ythrough the bridge resistor 34, thereby -unbalancing the lbridge so that an alternating-current component Will appear between the terminals C13.

kIn accordance with .the present invention, the desired attenuation is dependent on the unbalanced condition which is deliberately obtained by the circuit which includes the source of biasing potential 53. The source of potential 53 supplies current which ilows through resistors I9, 49 and 48. Thus, a voltage is-developed across the resistor I9 which is applied to the Ygrids of the vacuum tubes It and i1 so that the ,grid yof `one vacuum tube becomes, With respect to its cathode, more positive by ia certain amount and the grid of the other vacuum .tube becomes more negative yby an equal amount. Under such yconditions of operation, thecircuit arrangement operates to supply a signal current output or voltage vacross the diagonal CD of .the #bridge network, the magnitude of which will depend on the extent of unbalance. The amplifier 35 amplies this potential which vris transmitted through .the coupling capacitor :il so that la portion ,of this energy .is applied Ato the primary winding 43 of the transformer 44. Rectifier tubes 46 and 41 convert this signal energy -into direct current .75 which ows through the resistors `t9 and I9,

thereby developing in the resistor lava n'et 'ipotential which is less than and opposite in sense with respect to the potential Adeveloped therein due to the energy supplied by the bias battery 53. This therefore will reduce the signal output across the terminals CD, and automatic attenuation or volume control is provided; or the control bias may be derived from an'RC circuit in manner well known to those' skilled in the art. f

Fig. 2 shows the relation between grid voltage and plate current by means ofa characteristic curve. For balanced operation, both the vacuum tubes have a similar negative bias due .to the action of the self-biasing resistors 2I`and'22 so that they both operate at the point ll on the curve shown in Fig. 2. Y The application of potential from the bias source 53, however, displaces the operating point of one of the vacuum tubes, for example the vacuum tube I6, to theA point vI on the curve and the operatingpoint of the other vacuum tube to the point".I on the curve. From this itis apparent that any signal thereafter impressed upon the grids of the vacuum'tubes will produce unequal effects so. that A`unequal alternating-current components are developed across the resistors and 2l which are in the anode circuits of thevacuum tubes I6 and I'I. With greater and greater output signalV voltage, the more nearly the biasing voltage derived therefrom by the rectifier-,filter networkbalances the fixed bias of battery 53 or equivalent, with the result the operating points I and 2, Fig. 2, of tubes IB and I'I both shift toward the point Il. Thus it becomes apparent that there has been provided a circuit arangement whereby the magnitude of signal supplied to the amplifier tube 36 may be controlled. The vacuum tube 36 and those in the succeeding stages of ampliiication will operate in accordance with normal operating voltages, and hence the amplifier will not introduce into the signal output any component of substantial magnitude derived from the gaincontrol voltage.

In the description of the circuit arrangement shown in Fig. 1 of the drawings, it was stated that the primary winding II of the transformer III may be connected to a suitable source of seismic signals, such as a geophone I2. Where in seismic recording it is desired to have the seismograph make a sharp break in the trace, the gain of the amplifier is set to be at or near a maximum. In accordance with the present invention, the full effect of the bias potential 53 is permitted initially to appear across the resistor I9, thereby resulting in the maximum signal amplification or signal magnitude first appearing across the output of the amplifier AM which supplies the energy to the oscillographic elements in the seismic recording apparatus R.

It may therefore be assumed that in the application of the circuit of Fig. l, the direct traveling wave is picked up by the geophone and transmitted by the amplifier tube 36. The maximum signal energy having been impressed upon the grid of this vacuum tube results in a maximum amount of energy being supplied to the rectiers I6 and 41 which thereupon operate to reduce the effective bias across the resistor I9 by an appreciable amount, thereby bringing about a condition of operation corresponding to the reduction of the gain in the ordinary amplier to a relatively low value. Thus, the amplifier is prepared for reception of the rst reflected train of signals of greater amplitude as received by the geophone connected to the transformer III. These are then suppliedto the amplifier tube 36 and thesuc;l 'needing stages of amplification. The signal en- -erg'y of succeeding signals transmittedv to the amplifier, being less than the initial energy being transmitted in response to the direct wave, will cause the rectiers 46 and 4l to supply potential of decreasing values. As the potential developed by the vrectieis 46 and 41 decreases, the net potential across the resistor I9 increases, thereby returning the operating conditions of the vacuum tubes I6 and I'I to a greater condition of unbalance. This produces an effect corresponding to an increase in the gain of an amplifier in a conventional circuit, as far as the output of the amplifier AM and the recording apparatus R are concerned. Thus, for'the subsequent and more reduced amplitude waves, that is, lower inputsignal levelethe transmission energy level of the 'circuit comprising the vacuum tubes I6 and vI'I 'and the bridge network is increased so that substantially constant signal energy is transmitted to the recording apparatus R. 1 A modified arrangement is illustrated in Fig'. 3,

l wherein those parts corresponding to the vcom'- ponents shown in Fig. l have been given similar reference characters. It will be noted that this circuit arrangement does not employ the resistor vI8 since it is possible to develop alternating-current potential across the resistor I9, due to the coupling capacitors Maand I5, which corresponds vwith the alternating current developed `in. the secondary winding I3. The capacitors I4 and I5 actually serve as isolating capacitors to prevent the direct current supplied by the bias potential 53 from circulating through the transformer winding I3. The anodes of the vacuum tubes I6 and I 'I are not connected to a bridge network. The grid of the vacuum tube 36 is coupled by a capacitor 6B with a common juncture between the resistors 25 and 21, and a resistor 6I is included .between that ljuncture and ground. Since this common juncture between the resistors un der conditions of unbalanced operation of the vacuum tubes I6 and Il has an alternating-current potential relative to ground which is absent under balanced conditions, this potential may be applied by the coupling capacitor 60 directly upon the grid of the amplifier tube 36. In all other respects the circuit operation is the same.

While for the purpose of illustrating and describing the present invention there are shown in the drawings particular circuits, it is to be understood that the invention is not to be limited thereby since such variations may be made in the circuit arrangements in accordance with the principles set forth by the present invention as may be commensurate with the spirit and scope of the invention defined in the following claims.

What is claimed is:

1. A seismograph amplifier comprising a pair of vacuum tubes arranged in balanced circuit relation, grid-biasing means for applying between the respective signal grids and cathodes of said tubes a bias for operation of each tube at the same point on their respective characteristic curves. means including a resistor having a mid-portion connected to the cathodes of said tubes and the respective end portions connected to the signal grids of said tubes, a source of direct current potential connected to the respective end portions of said resistor to bias said tubes respectively in opposite senses and away from said point of said characteristic curves for raising the transmission level of seismic signals to a predetermined value by unbalancing said circuit relation, means including a second network responsive Yto output signalsfrom said amplifier above a .predetermined Vminimum levelfor applying a direct current biasing rpotential in opposition to said source of direct current potential simultaneously to ychange the bias on said tubes in directions 'for operation of each nearer said point on said characteristic curves to control the amplitude of the output signals;

2. A seismograph amplifier comprising a pair of lvacuum tubes arranged in balanced circuit relation, gridiasing means forapplying vbetween the respective signal grids and cathodes of said tubes :abias'for operation of each tube at the same point on their respective characteristic curves, means including a resistor .having a mid-portion connected to the cathodes of said tubes and there- =spective end `portions connected to thesignal grids of said tubes, a source of direct current potential connected to the respective end portions of said resistor to bias said -tubes respectively in opposite senses and away from said point of said characteristiccurves for raising the transmission level 8 end*A of which is connected vto ground, and means including an amplifier having an input circuit connected to the other end of said second diagonaland responsive to output signals from said amplifier above .a predetermined minimum level for applying a vdirect current biasing potential in opposition to said source of direct current potential simultaneously to change the bias on said tubes in directions for operation of each nearer said point on said characteristic curves to control the amplitude of the output signals, said bridge being 'adjustable .to balance out any difference in anode vcurrents due to characteristic curves for the respective tubes of different amplitude.

EDWIN J. SHIMEK.

REFERENCES CITED The following references are of record in the iil'e of this patent:

UNITED STATES PATENTS Number Name Date 2,244,695 Hathaway June 10, 1941 2,251,594 Mayne Aug. 5, 1941 '2,255,683 Singer Sept. 9, 1941 2,294,200 Norman Aug. 25, 1942 2,318,795 Peters May 11, 1943 2,352,825 Fay July 4, 1944 '2,354,420 Minton July 25, 1944 2,390,850 Singer Dec. 11, 1945 

